This invention relates to a broadband antenna unit and, more particularly, to a broadband antenna unit included in a mobile equipment terminal and an antenna element for use in it.
An ultra wideband (UWB) technology means an ultra wideband radio technology like its name and is defined as any radio technology having a spectrum that occupies a bandwidth greater than 25 percent of the center frequency, or a bandwidth of at least 1.5 GHz. In a word, the UWB technology is technology for communicating using short pulses (normally each having a pulse width of 1 ns or less) of ultra wideband so as to start a revolution in radio technology.
A crucial difference between a conventional radio technology and the UWB technology is the presence or absence of a carrier wave. The conventional radio technology modulates a sinusoidal wave having a frequency called the carrier wave using various methods to transmit and receive data. On the other hand, the UWB technology does not use the carrier wave. In the manner which is written in definition of the UWB technology, the UWB technology uses the short pulses of the ultra wideband.
Like its name, the UWB technology has a frequency band of the ultra wideband. On the other hand, the conventional radio technology has only a narrow frequency band. This is because it is possible, with the narrow frequency band, to effectively utilize electric waves. The electric waves are finite resources. The reason why the UWB technology is widely noticed in spite of the ultra wideband is output energy of each frequency. The UWB technology has a very small output at each frequency although a frequency band is wide. Inasmuch as the output of the UWB technology has such a magnitude as to be covered with noises, the UWB technology reduces interference with other wireless spectra. In the United States, the Federal Communications Commission (FCC) has mandated that UWB radio transmissions can legally operate in a range from 3.1 GHz to 10.6 GHz, at a limited transmit power of −4.1 dBm/MHz.
In addition, antennas basically use a resonance phenomenon. The antenna has a resonance frequency which is determined by its length. However, it is difficult for the UWB including a lot of frequency components to make the antenna for UWB resonate. Accordingly, the wider the frequency band of the electric wave to be transmitted is, the more difficult it is to make a plan or design for the antenna for UWB.
Taiyo Yuden Co. Ltd. has successfully developed a very miniaturized ceramic chip antenna having a size of 10×8×1 mm for ultra wideband applications. Since UWB technology was released by the FCC commercial use, it has been hailed as the short-range wires-communication standard of the future. For one thing, it promises to simultaneously provide a high data rate and low power consumption. By sending very low-power pulses below the transmission-noise threshold, UWB also avoids interference. By developing the antenna, it has become the responsibility of the wireless industry to help UWB make the transition from military applications to widespread commercial use for connecting at a very high speed data between digital devices such as PDP (plasma display panel) television, a digital camera, or the like.
In addition, such a UWB antenna can be used for various purposes such as Bluetooth (registered trademark), wireless LAN (local area network), or the like.
Bluetooth (registered trademark) technology is a cutting-edge open specification that enables short-range wireless connections between desktop and notebook computers, handhelds, personal digital assistants, mobile phones, camera phones, printers, digital cameras, handsets, keyboards and even a computer mouse. Bluetooth wireless technology uses a globally available frequency band (2.4 GHz) for worldwide compatibility. In a nutshell, Bluetooth technology unplugs your digital peripherals and makes cable clutter a thing of the past.
The wireless LAN is an LAN using a transmission path except for a wire cable, such as electric waves, infrared rays, or the like.
Various broadband antenna devices are already known in the art. By way of example, JP 2003-273638 A discloses a wideband antenna device with which interference to be exerted by an unwanted frequency band or a frequency band out of a target is reduced by forming the wideband antenna device matched with target frequency characteristics. According to JP 2003-273638 A, the wideband antenna device comprises a flat conductive ground plate and a flat radiation conductor standing up above a plane of the flat conductive ground plate in a direction to intersect the flat conductive ground plate. The wideband antenna device has a feeding point on or near an outer peripheral portion of the flat radiation conductor. The flat radiation conductor has one or more notches formed by cutting a part of the flat radiation conductor.
In addition, JP 2003-283233 A discloses a wideband antenna device with a wide band and a small size that counters the problems in costs, usage purposes or mounting on equipment and that is capable of cutting manufacturing costs. According to JP 2003-283233 A, the wideband antenna device comprises a flat conductive ground plate and a polygonal flat radiation conductor standing up above a plane of the flat conductive ground plate in a direction to intersect the flat conductive ground plate. The polygonal flat radiation conductor has a top which is used as a signal feeding point.
Furthermore, JP 2003-304114 A discloses a wideband antenna device which uses a plate-shaped radiation conductor as a radiation conductor and which can be made more compact. According to JP 2003-304114 A, the wideband antenna device comprises a flat conductive ground plate and a flat radiation conductor standing up above a plane of the flat radiation ground plate in a direction to intersect the flat conductive ground plate. In a state where the flat radiation conductor stands up above the plane of the flat conductive ground plate, the flat radiation conductor comprises a plurality of conductive portions so as to be arranged in the direction to intersect the flat conductive ground plate. Through a low conductivity member having conductivity of almost 0.1 [/Ωm] or more and 10.0 [/Ωm] or less, the plurality of conductive portions are connected.
In the wideband antenna devices disclosed in the above-mentioned JP 2003-273638 A, JP 2003-283233 A, and JP 2003-304114 A, the flat radiation conductor stands up above the plane of the flat conductive ground plate in the direction to intersect the flat conductive ground plate. Therefore, the wideband antenna devices are high in profile and it is difficult to include the wideband antenna device in a portable equipment terminal. In addition, in the above-mentioned JP 2003-304114 A, the disclosed wideband antenna device has a low limit frequency of 2.32 GHz and cannot support a frequency lower than the low limit frequency.
A thin-type wideband antenna device is disclosed in JP 2003-304115 A which corresponds to U.S. Pat. No. 6,914,561 issued to Shinichi Kuroda et al. According to JP 2003-304115 A, the thin-type wideband antenna device includes a reference conductor (conductive ground plate) and a radiation conductor that are connected with a feeder line for transmitting power, at least parts of which are disposed so as to face each other. Interposed between the parts that the reference conductor and the radiation conductor face each other, a substance has conductivity which is about 0.1 [/Ωm] through 10 [/Ωm] in the operational radio frequency.
However, the thin-type wideband antenna device disclosed in JP 2003-304115 A is disadvantageous in that an operable band is narrow.
On the other hand, an ultra wideband (UWB) antenna unit which is capable of widening the band and which is capable of improving a frequency characteristic has already been proposed in JP 2005-94437 A which corresponds to U.S. Pat. No. 7,081,859 issued to Akira Miyoshi et al. According to JP 2005-94437 A, the UWB antenna unit comprises an upper dielectric, a lower dielectric, and a conductive pattern sandwiched therebetween. The conductive pattern has a feeding point at a substantially center portion of a front surface. The conductive pattern comprises a reversed triangular portion having a right-hand taper part and a left-hand taper part which widen from the feeding point at a predetermined angle toward a right-hand side surface and a left-hand side surface, respectively, and a rectangular portion having a base side being in contact with an upper side of the reversed triangular portion. In addition, the feeding point of the conductive pattern is electrically connected to a ground plate which extends in a plane similar to that of the conductive pattern (a radiation element).
Inasmuch as the UWB antenna unit disclosed in JP 2005-94437 A has a usable frequency band which lies between about 4 GHz and about 9 Hz. Therefore, the usable frequency band is narrow.
Various thin UWB antennas which cover a UWB band between 3.1 GHz and 10.6 GHz are proposed in the art. By way of example, an elliptically shaped ring broadband antenna is reported by Satoshi Hattori et al in a first paper contributed to 2005 National Convention of the Institute of Electronics, Information and Communication Engineers of Japan as Paper No. B-1-104, Osaka, Japan, May, 2005, under the title of “An Elliptically Shaped Ring Broadband Antenna.” In the elliptically shaped ring broadband antenna reported in the first paper, an elliptically shaped radiation element has an outside diameter in a major axis direction of 24 mm and a ground plate has a square with a side of 45 mm.
Another elliptically shaped ring broadband antenna is reported by Satoshi Hattori et al in a second paper contributed to 2005 Communication Society Convention of the Institute of Electronics, Information and Communication Engineers of Japan as Paper No. B-1-82, Hokkaido, Japan, September, 2005, under the title of “An Elliptically Shaped Ring Broadband Antenna—Part II.” The elliptically shaped ring broadband antenna reported in the second paper comprises a ground plate having a semi-elliptically shaped upper edge.
Still another elliptically shaped ring broadband antenna is reported by Satoshi Hattori et al in a third paper contributed to 2006 National Convention of the Institute of Electronics, Information and Communication Engineers of Japan as Paper No. B-1-165, Tokyo, Japan, May, 2006, under the title of “An Elliptically Shaped Ring Broadband Antenna—Part III.” The elliptically shaped ring broadband antenna reported in the third paper comprises a ground plate having a lower portion where both side corner portions are deleted with a central portion left. With this structure, it is possible to improve a gain in a +z direction at or more than a frequency of 9 GHz.
The elliptically shaped ring broadband antennas reported in the first through the third papers cover the UWB band between 3.1 GHz and 10.6 GHz. However, it is difficult to cover a frequency band lower than the UWB band, for example, a frequency band (2.45 GHz band) for use in the wireless LAN, a frequency of 1.575 GHz for use in a global positioning system (GPS), or a frequency band (e.g. 2.1 GHz band) for use in a cellular telephone.
In addition, various antenna devices included in portable wireless terminals are already known in the art. By way of example, a dual band built-in antenna device is disclosed in JP 2002-185238 A which corresponds to U.S. Pat. No. 6,535,170 issued to Masatoshi Sawamura et al. The dual band built-in antenna device disclosed in JP 2002-185238 A is operable in a first frequency band and a second frequency band. The dual band built-in antenna device comprises a ground plane comprising a ground member, a first inverted-L line antenna element for the first frequency band, and a second inverted-L antenna element for the second frequency band. The first and the second inverted-L line antenna elements are so constructed that the elements are extended in respective directions further away from each other as the antenna elements extend further from a starting position set in proximity to a power feed point within a plane parallel to the ground plane. The dual band built-in antenna device further comprises a matching circuit shared with the first and the second inverted-L line antenna elements.
In JP 2002-185238 A, as mobile wireless terminals comprising such dual band built-in antenna devices, following multiplex terminals are intended (targeted). A multiplex terminal which can jointly use PDC (Personal Digital Cellular) operation on 800 MHz band and PHS (Personal Handyphone System) operation on 1.9 GHz has been made commercially availably in Japan. Another multiplex terminal capable of jointly using GSM (Global System for Mobile Communication) operation on 900 MHz band and DCS (Digital Communication System) operation on 1.8 GHz has also been on the market in Europe and Asian countries. Moreover, another multiplex terminal which can operate on both AMPS (Advanced Mobile telephone Service) using 800 MHz band and PCS (Personal Communication Service) using 1.9 GHz band has been on sale in the United States.
JP 11-68453 A proposes a composite antenna which has a small external size and which can easily obtain a desired feeding point impedance. The composite antenna disclosed in JP 11-68453 A comprises plural nearly U-shaped folded antennas corresponding to plural frequency bands. Each U-shaped folded antenna includes a main element having one end as a feeding point and a sub-element folded from another end of the main element. The sub-element has an opened end. The main elements of the U-shaped folded antenna are integrated to reduce the external size of the composite antenna. In JP 11-68453 A, a low frequency band is 860 MHz band while a high frequency band is 1900 MHz band.
The antenna devices disclosed in JP 2002-185238 A and JP 11-68453A only cover the low frequency band between 800 MHz and 900 MHz and the high frequency band between 1.8 GHz and 2.0 GHz. Accordingly, the antenna devices disclosed in JP 2002-185238 A and JP 11-68453A are disadvantageous in that it is impossible to cover the above-mentioned UWB band.